1. Field of the Invention
The present invention relates to a parallel acquisition system configured using a reference matched filter and applied to the receiver of a spread spectrum system for packet radio data communications in a multiple access environment.
2. Description of the Prior Art
In order to operate efficiently a direct-sequence spread spectrum system in a multiple access environment, it is essentially required to synchronize a pseudonoise sequence received in the receiver of the system with a pseudonoise sequence locally generated in the receiver.
The synchronization carried out in such a spread spectrum system mainly involves two steps; an acquisition and a tracking.
FIG. 1 is a block diagram illustrating a conventional parallel acquisition system using no reference matched filter. In FIG. 1, the reference numeral 11 denotes a matched filter, and the reference numeral 12 denotes a decision unit.
As shown in FIG. 1, the conventional parallel acquisition system includes N matched filters 11 in order to process a reception signal received therein in a parallel manner. By virtue of such a parallel processing for a reception signal, the parallel acquisition system can reduce the average acquisition time, as compared to serial acquisition systems. The decision unit 12 determines whether or not the resultant signals obtained by the parallel processing have a higher value than a critical value. Based on the result of the determination, the decision unit 12 executes a coincidence detection mode.
Although the above-mentioned parallel acquisition system having no reference matched filter reduces the average acquisition time as compared to serial acquisition systems, it still has a problem in that the throughput of the entire system is degraded.
On the other hand, FIG. 2 is a block diagram illustrating a conventional serial acquisition system. In FIG. 2, the reference numeral 21 denotes multipliers, 22 matched filter correlation detectors, 23 square-law detectors, 24 an adder, 25 a comparator, and 26 a decision unit.
When a reception signal is received in the serial acquisition system, it is splitted into its in-phase component and its quadrature component which, in turn, pass through the adders 21. An output from each adder 21 is sent to an associated one of the matched filter correlation detectors 22 which, in turn, multiplies the reception signal by a pseudonoise sequence, thereby calculating correlations of those values. That is, each matching filter correlation detector 22 outputs a value of 1 when the correlations of the input values are the same while outputting a value of 0 when the correlations of the input values are different. Each matched filter correlation detector 22 has a configuration shown in FIG. 3. In the configuration shown in FIG. 3, a reception signal passes through an integrator 31 which, in turn, sends its output to a code weighter 34 via a tapped delay line 32. The code weighter 34 also receives an output from a pseudonoise sequence generator 33 and calculates a code weight based on the reception signal. The code weighter 34 then adds the calculated code weight to the signal received from the tapped delay line 32. Thus, the code weight 34 outputs the resultant signal as an output of the matched filter correlation detector.
Output signals from the matched filter correlation detectors 22 are sent to the associated square-law detectors 23 and then added to each other in the adder 24. An output signal from the adder 24 is compared with a detection critical value in the comparator 25. When the output signal from the adder 24 is lower than the detection critical value, the above procedure is executed again from the step of inputting a reception signal. When the output signal from the adder 24 is not lower than the detection critical value, the decision unit 26 executes a coincidence detection mode operation.
However, the above-mentioned serial acquisition system requires a lengthened average acquisition time, thereby increasing an initial communication disable time. As a result, performance degradation occurs. That is, the conventional serial acquisition system using matched filters obtains a low throughput in a multiple access environment.